MINOR: Fix needless GC + Result time unit in JMH

Fixes two issues with the JMH benchmark example:
* Trivial: The output should be in `ops/ms` for readability
reasons (it's in the millions of operations per second)
* Important: The benchmark is not actually measuring the
LRU Cache performance as most of the time in each run is
wasted on concatenating `key + counter` as well as
`value + counter`. Fixed by pre-generating 10k K-V pairs
(100x the cache capacity) and iterating over them. This
brings the performance up by a factor of more than 5 on
a standard 4 core i7 (`~6k/ms` before goes to `~35k/ms`).

Author: Armin Braun <me@obrown.io>

Reviewers: Bill Bejeck <bbejeck@gmail.com>, Guozhang Wang <wangguoz@gmail.com>, Ismael Juma <ismael@juma.me.uk>

Closes #2903 from original-brownbear/fix-jmh-example

jmh-benchmarks/src/main/java/org/apache/kafka/jmh/cache/LRUCacheBenchmark.java

@@ -17,9 +17,11 @@
 
 package org.apache.kafka.jmh.cache;
 
+import java.util.concurrent.TimeUnit;
 import org.apache.kafka.common.cache.LRUCache;
 import org.openjdk.jmh.annotations.Benchmark;
 import org.openjdk.jmh.annotations.Level;
+import org.openjdk.jmh.annotations.OutputTimeUnit;
 import org.openjdk.jmh.annotations.Scope;
 import org.openjdk.jmh.annotations.Setup;
 import org.openjdk.jmh.annotations.State;
@@ -35,25 +37,39 @@ import org.openjdk.jmh.runner.options.OptionsBuilder;
  * http://hg.openjdk.java.net/code-tools/jmh/file/tip/jmh-samples/src/main/java/org/openjdk/jmh/samples/
  */
 @State(Scope.Thread)
+@OutputTimeUnit(TimeUnit.MILLISECONDS)
 public class LRUCacheBenchmark {
 
+    private static final int DISTINCT_KEYS = 10_000;
+
+    private static final String KEY = "the_key_to_use";
+
+    private static final String VALUE = "the quick brown fox jumped over the lazy dog the olympics are about to start";
+
+    private final String[] keys = new String[DISTINCT_KEYS];
+
+    private final String[] values = new String[DISTINCT_KEYS];
+
     private LRUCache<String, String> lruCache;
 
-    private final String key = "the_key_to_use";
-    private final String value = "the quick brown fox jumped over the lazy dog the olympics are about to start";
     int counter;
 
-
     @Setup(Level.Trial)
-    public void setUpCaches() {
+    public void setUp() {
+        for (int i = 0; i < DISTINCT_KEYS; ++i) {
+            keys[i] = KEY + i;
+            values[i] = VALUE + i;
+        }
         lruCache = new LRUCache<>(100);
     }
 
     @Benchmark
     public String testCachePerformance() {
         counter++;
-        lruCache.put(key + counter, value + counter);
-        return lruCache.get(key + counter);
+        int index = counter % DISTINCT_KEYS;
+        String hashkey = keys[index];
+        lruCache.put(hashkey, values[index]);
+        return lruCache.get(hashkey);
     }
 
     public static void main(String[] args) throws RunnerException {